Vaccine compositions often include immunological adjuvants to enhance immune responses. For example, Complete Freund's adjuvant (CFA) is a powerful immunostimulatory agent that has been successfully used with many antigens on an experimental basis. CFA includes three components: a mineral oil, an emulsifying agent, and killed mycobacteria, such as Mycobacterium tuberculosis. Aqueous antigen solutions are mixed with these components to create a water-in-oil emulsion. Although effective as an adjuvant, CFA causes severe side-effects, including pain, abscess formation and fever, primarily due to the presence of the mycobacterial component. CFA, therefore, is not used in human and veterinary vaccines.
Muramyl dipeptide (MDP) is the minimal unit of the mycobacterial cell wall complex that generates the adjuvant activity observed with CFA. See, e.g., Ellouz et al., Biochem. Biophys. Res. Commun. (1974) 59:1317. Several synthetic analogs of MDP have been generated that exhibit a wide range of adjuvant potency and side-effects. For a review of these analogs, see, Chedid et al., Prog. Allergy (1978) 25:63. Representative analogs of MDP include threonyl derivatives of MDP (Byars et al., Vaccine (1987) 5:223), n-butyl derivatives of MDP (Chedid et al., Infect. Immun. 35:417), and a lipophilic derivative of a muramyl tripeptide (Gisler et al., in Immunomodulations of Microbial Products and Related Synthetic Compounds (1981) Y. Yamamura and S. Kotani, eds., Excerpta Medica, Amsterdam, p. 167).
One lipophilic derivative of MDP is N-acetylmuramyl-L-alanyl-D-isogluatminyl-L-alanine-2-(1′-2′-dipalmitoyl-sn-glycero-3-huydroxyphosphoryloxy)-ethylamine (MTP-PE). This muramyl tripeptide includes phospholipid tails that allow association of the hydrophobic portion of the molecule with a lipid environment while the muramyl peptide portion associates with the aqueous environment. Thus, the MTP-PE itself is able to act as an emulsifying agent to generate stable oil-in-water emulsions. MTP-PE has been used in an emulsion of 4% squalene with 0.008% TWEEN 80™, termed MTP-PE-LO (low oil), to deliver the herpes simplex virus gD antigen with effective results (Sanchez-Pescador et al., J. Immunol. (1988) 141:1720-1727), albeit poor physical stability. Recently, MF59, a safe, highly immunogenic, submicron oil-in-water emulsion which contains 4-5% w/v squalene, 0.5% w/v TWEEN 80™, 0.5% SPAN 85™, and optionally, varying amounts of MTP-PE, has been developed for use in vaccine compositions. See, e.g., Ott et al., “MF59—Design and Evaluation of a Safe and Potent Adjuvant for Human Vaccines” in Vaccine Design: The Subunit and Adjuvant Approach (Powell, M. F. and Newman, M. J. eds.) Plenum Press, New York, 1995, pp. 277-296.
Interferons and other cytokines, such as IFN-1, are normally expressed at low levels but are induced to high levels of expression by a number of stimuli such as viral and bacterial infection. It is now believed that the viral product responsible for cytokine production is double-stranded RNA (dsRNA). See, e.g., Majde, J. A., J. Interfer. Cytokine Res. (2000) 20:259-272. In fact, proinflammatory cytokines induced by viral dsRNA are now thought to be largely responsible for the flu-like symptoms caused by bacterial and viral infections, such as fever, fatigue, drowsiness and muscle aches. IFN-1 production, stimulated by delivery of dsRNA, has been reported to display adjuvant activity. See, e.g., Le Bon et al., Immunity (2001) 14:461-470.
Despite the presence of adjuvants, conventional vaccines often fail to provide adequate protection against the targeted pathogen. Accordingly, there is a continuing need for effective vaccine compositions which include safe and non-toxic adjuvants.